Strategic Farming: Field Notes talks forage and pasture management

By Liz Stahl, Extension Educator – Crops, Anthony Hanson, Extension Educator – Integrated Pest Management, and Troy Salzer, Extension Educator – Ag Production Systems

Forage pests, the status of the forage crop, and pasture management were topics of discussion on the May 28 University of Minnesota Extension Strategic Farming: Field Notes program. Anthony Hanson, UMN Extension Educator in Integrated Pest Management, and Troy Salzer, UMN Extension Educator – Ag Production Systems, discussed these issues and more, with moderator UMN Extension Crops Educator, Dave Nicolai.

Status of the alfalfa crop

Although there was enough snow cover in parts of northern MN this past winter, overall, we didn’t have a lot of snow cover over much of the state. Times of little snow cover often correlated to our coldest spells, resulting in some alfalfa winter kill, especially with older stands. Considering these factors, some fields might be looking a little rough this spring. Be sure to evaluate stands, making sure you have enough to meet your needs.

See “Assessing alfalfa winter injury” for more details on assessing stands and management options.

Alfalfa weevil

Alfalfa weevil, a key pest in alfalfa, overwinters as an adult in plant debris. When alfalfa doesn’t overwinter well, alfalfa weevil usually doesn’t either. Where we didn’t have much snow cover this year, overwintering mortality should have been higher than normal. Where there was a bit of snow cover, however, this would have offered some protection, increasing the risk of alfalfa weevil pressure this year. Overall, Hanson reports alfalfa weevil numbers to date have been relatively low in west-central Minnesota.

Considering the pest’s life cycle, we are not out of the woods yet. Overwintering adults will lay eggs in the stems of plants in late April/early May. Larvae that hatch from the eggs will feed on leaves, skeletonizing them. As larvae grow, the amount of foliage they consume will greatly increase. The larvae go through four different instars, then into a pupae and cocoon, eventually turning into adults. Although adult beetles will feed for a short time, they do not cause economic damage - the larvae are instead the primary cause of damage. Treating adults in a field is not likely to give a return on investment or prevent infestations the next year.

Forecast maps using degree-days are available for alfalfa weevil, and current forecasts have pupation occurring across central Iowa as of May 30 (Fig. 1). 7-day forecasts have this life stage occurring across central MN. Keep in mind these prediction models are only for the eastern strain of alfalfa weevil. It is not known how prevalent the western strain is in the state, but they tend to have a 1-2 week lag in development compared to the eastern strain. This may explain the prolonged weevil damage we’ve seen in past years, so scouting likely should continue until mid-June at this current pace.

Figure 1. Eastern strain alfalfa weevil degree-days as of May 3, 2025. Updated forecast maps can be viewed at https://vegedge.umn.edu/alfalfa-weevil.

Scouting and treating for alfalfa weevil

Sweep nets are often used when scouting for pests, but they may not be the most accurate when scouting for alfalfa weevil. Hanson carries a 5-gallon bucket with him and shakes out 5 plants per location at 5 locations in a field (for a total of 30 plants) into the bucket to dislodge weevils from the plants to get a count.

Mowing early causes weevil mortality. Weevils can, however, be concentrated in windrows, so be sure to check under windrows when scouting. When to treat should be based on crop height, price, and the number of larvae present per plant.

Further details on scouting tips, treatment thresholds, and insecticide options can be found at “Alfalfa weevil season begins in 2024”.

 
Table 1. Economic thresholds for alfalfa weevil based on hay value. PHI = pre-harvest interval. Reviewed by Pellissier et al. 2017 and modified from NDSU Extension.

		Hay value ($/ton)
		Treatment	$50	$75	$100	$125	$150	$175
Stem height	Management tactics	cost/acre	Average larvae per stem
10-15 inches (mid-vegetative)	Long or short PHI insecticide	$7 $8 $9 $10 $11 $12	3.6 4.1 4.7 5.3 5.9 6.4	2.2 2.6 3.0 3.4 3.7 4.1	1.5 1.8 2.1 2.4 2.7 3.0	1.1 1.4 1.6 1.8 2.1 2.3	0.9 1.1 1.2 1.4 1.6 1.8	0.7 0.8 1.0 1.2 1.3 1.5
16-20 inches (late vegetative)	Short PHI insecticide or mow early	$7 $8 $9 $10 $11 $12	3.8 4.4 4.9 5.5 6.1 6.7	2.4 2.8 3.2 3.6 4.0 4.4	1.8 2.1 2.4 2.6 2.9 3.2	1.4 1.6 1.8 2.1 2.3 2.5	1.1 1.3 1.5 1.7 1.9 2.1	0.9 1.1 1.2 1.4 1.6 1.7
>20 inches* (early bud)	Mow early	$7 $8 $9 $10 $11 $12	4.0 4.6 5.2 5.8 6.3 6.9	2.7 3.1 3.5 3.8 4.2 4.6	2.0 2.3 2.6 2.9 3.2 3.5	1.6 1.8 2.1 2.3 2.5 2.8	1.3 1.5 1.7 1.9 2.1 2.3	1.2 1.3 1.5 1.6 1.8 2.0

There are not very many pesticide options for use in alfalfa. If you have any existing stocks of chlorpyrifos products registered for this year, these can be used on alfalfa until June 30, 2025. Old chlorpyrifos products with expired registrations cannot be used this year. Check the MN Dept. of Ag. registered product list for more information. There may be products with new labeling that can be used after this time provided they comply with updated EPA guidelines. For more details see Chlorpyrifos products and their use in Minnesota. One common trade name farmers often ask about is Lorsban. That product was not registered for this year and cannot be applied to field crops this year even before June 30.

Table 2. Conventional insecticide options for alfalfa weevil. This is not an exhaustive list and does not endorse specific products or trade names. Remember existing stocks of chlorpyrifos products are only available for use through June 30, 2025.

Group	Class	Active ingredient	Trade Names
1A	Carbamate	Methomyl	Lanate
		Carbaryl*	Sevin
1B	Organophosphate	Malathion†	Malathion
		Phosmet†	Imidan
		Chlorpyrifos**	Pilot 4E, Warhawk, Lorsban (not registered in 2025)
3A	Pyrethroid‡	Alpha-cypermethrin‡	Fastac Mustang-Maxx,
		Beta-cyfluthrin‡	Baythroid
		Cyfluthrin‡	Tombstone
		Gamma-cyhalothrin‡	Declare, Proaxis
		Lambda-cyhalothrin‡	Warrior and generics
		Permethrin‡	Arctic, Permethrin, others
		Zeta-cypermethrin‡	Mustang Maxx
22	Oxadiazine	Indoxacarb	Steward

As far as insecticide options, pyrethroid resistance is a concern. If you suspect pyrethroid resistance in a field, let us know. The University is hoping to do some screening for pyrethroid resistance and is interested in collecting samples to test from suspect populations. Contact Anthony Hanson (hans4022@umn.edu) if you suspect you had a field with a pyrethroid failure.

Pea aphids

Pea aphids have been found in the region, but not at a concerning level. Once you reach 50 or more pea aphids per stem on 10-inch-tall alfalfa, or 100 on 20-inch-tall alfalfa, treatment is justified.

Potato leafhopper

Potato leafhopper is usually a later-season pest, particularly if conditions turn dry and the crop is stressed. Potato leafhoppers migrate up from the Gulf Coast states each spring. First-year alfalfa plantings tend to be at higher risk of injury. Resistant varieties can handle slightly higher levels of potato leaf hopper.

For more information on scouting and treatment thresholds for alfalfa insect pests, check out Alfalfa insects: What to look for and how to scout.

Forage grasses and pasture

Salzer discussed how grasses with longevity, like orchardgrass, can be highly productive but mature earlier than many alfalfa varieties. Meadow fescue, or meadow brome, may match better with alfalfa cutting times, and offer an advantage from a quality standpoint, particularly for digestibility of the fiber fraction. High-quality grass along with alfalfa, offers producers benefits in how it will feed to the livestock.

This season, pastures have been slow to regrow, particularly in southeastern MN, although recent rains have helped. Pale, light-green plants are an indication of a shortage of N, so a timely application of N may help rejuvenate pastures in some areas.

If your pasture species aren’t aggressively and actively growing, weeds can become a problem. Fertilizing can help regenerate growth, as well as managing the plants with timely harvesting. Using a burn-down herbicide to kill weeds and then seeding to introduce a species you want can help with productivity. Consider a nurse crop, like oats, to allow for some grazing, while protecting new seedlings. Grazing and pasture management for cattle provides more details on how to achieve the most productivity from our pasture.

Listen to the full program through your preferred podcasting platform or at the Strategic Farming: Field Notes website.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.